With respect to rotatable bodies such as various machine tools, it is necessary to correct dynamic imbalance caused by vibration, etc. at times of rotation. Until now, dynamic imbalance has been corrected by measuring and obtaining the amount and phase angle of imbalance, with use of a balance measuring device; and then shedding off or adding the amount equivalent to the amount of the imbalance. However, correction of such dynamic imbalance has been a fixed balance correction made for a certain rotation speed designated for measurement. Therefore, although balance immediately after correction would be fine, dynamic imbalance would be created again in instances where there is remarkable change in rotation speed or change in the weight of the rotatable body due to tool changing or the like, since there would also be change in the balance corrective value. That is, adaptation to changes in rotation conditions has not been possible, thus requiring balance correction every time there is change in rotation conditions.
In this regard, Patent Literature 1 basically provides, as a balancer, an automatic dynamic balancer comprising: a casing capable of being driven to rotate together with a rotatable body; storage chambers drilled inside the casing and arranged in a radial manner toward a circumference created in the rotating direction; outer weights housed in the storage chambers, respectively, with extra space serving as a buffer also provided therein; inner-weight-holding spaces drilled in the outer weights, respectively; inner weights held in the inner-weight-holding spaces, respectively, with extra space serving as a buffer also provided therein; and a cover for the casing. Also, formed on the outer weight at its surface which defines its outer outline, is a sliding surface for being biased toward the storage chamber, the formation being due to centrifugal force created by rotation of the rotatable body; and formed in the storage chamber at its surface which defines its inner outline, is a guiding plane for guiding in a sliding manner the sliding surface of the outer weight, toward the bottom of the storage chamber. Thus, in rotatable bodies such as various machine tools, even if there is change in rotation conditions such as change in rotation speed or in the amount of rotation, the change would be traced, thereby enabling automatic correction of dynamic imbalance caused by vibration, etc. during rotation.